The aim of our project is to learn more about the structure and organization of immunoglobulin genes and mRNAs in the mouse. To this end, we have cloned a number of kappa mRNAs and different heavy chain mRNAs and characterized the clones. We are using probes made from these clones to investigate the number and arrangement of Ig genes in the germline state and in various Ig-synthesizing cells. In particular, for the heavy chain system, we are investigating the mechanism of VH-C micron joining by analysis of a cloned active C micron gene and the mechanism of CH switching. Analysis of other clones within the CH locus is underway.